In OneNote 2010 setting a desired image as background could be great, unfortunately it does not support it directly. But still if you want to give your notesheet a better look by setting image background. You can do it by following a simple procedure.
Launch OneNote 2010 and move to the desired notesheet in which you want to insert image as background.
Now navigate to Insert tab and click Picture.
It will bring-up Insert Picture dialog, now choose the desired image and click Insert.
It will insert image in the notesheet as an overlay image which will conceal all the existing content of the notesheet. Now resize the image to cover up the whole notesheet area.
Right-click the image and hit Set Picture as Background.
Upon click, all the hidden content; images, text, tables etc will show up and you can start taking notes again, as the image is now fixed and set as background.